Epithelial mucosal surfaces are lined with fluids whose volume and composition are precisely controlled. In the airways, a thin film of airway surface liquid helps protect mammalian airways from infection by acting as a lubricant for efficient mucus clearance (Chmiel et al., Respir. Res. 4:8 (2003); Knowles et al., J. Clin. Invest. 109:571 (2002)). This layer moves cephalad during mucus clearance and excess liquid that accumulates as two airways converge is eliminated by Na+-led airway surface liquid absorption with Na+ passing through the epithelial Na+ channel (ENaC) (Knowles et al., J. Clin. Invest, 109:571 (2002)). How ENaC activity is sensed and controlled by the airways is poorly understood. However, there is evidence that reporter molecules in the airway surface liquid can serve as volume sensing signals whose dilution or concentration can alter specific cell surface receptors which control ion transport rates to either absorb or secrete airway surface liquid as needed (Chambers et al., Respir. Physiol. Neurobiol, 159:256 (2007)). ENaC must be cleaved by intracellular furin-type proteases and/or extracellular channel activating proteases (CAPs) such as prostasin to be active and to conduct Na+ (Planes et al., Curr. Top. Dev. Biol. 78:23 (2007); Rossier, Proc. Am. Thorac. Soc. 1:4 (2004); Vallet et al., Nature 389:607 (1997); Chraibi et al., J. Gen. Physiol. 111:127 (1998)). ENaC can also be cleaved and activated by exogenous serine proteases such as trypsin, an action that is attenuated by the protease inhibitor aprotinin (Vallet et al., Nature 389:607 (1997)). When human bronchial epithelial cultures are mounted in Ussing chambers where native airway surface liquid is washed away, ENaC is predominantly active, suggesting that cell attached proteases are predominant (Bridges et al., Am. J. Physiol. Lung Cell. Mol. Physiol. 281:L16 (2001); Donaldson et al., J. Biol. Chem. 277:8338 (2002)). In contrast, under thin film conditions, where native airway surface liquid is present, ENaC activity is reduced, suggesting that airway surface liquid contains soluble proteases inhibitors (Myerburg et al., J. Biol. Chem. 281:27942 (2006); Tarran et al., J. Gen. Physiol. 127:591 (2006)).
The Palate Lung and Nasal epithelial Clone (PLUNC) family are secreted proteins that are subdivided into short (SPLUNCs) and long (LPLUNCs) members which contain either one or two domains respectively (Bingle et al., Biochim. Biophys. Acta 1493:363 (2000); Weston et al., J. Biol. Chem. 274:13698 (1999)). The original PLUNC gene which is now called SPLUNC1 comprises up to 10% of total protein in the airway surface liquid and can readily be detected in both nasal lavage and tracheal secretions (Bingle, C. D., and Craven, C. J. (2002) PLUNC: a novel family of candidate host defense proteins expressed in the upper airways and nasopharynx Hum Mol Genet 11, 937; Campos, M. A., et al. (2004) Purification and characterization of PLUNC from human tracheobronchial secretions Am J Respir Cell Mol Biol 30, 184; Lindahl, M., Stahlbom, B., and Tagesson, C. (2001) Identification of a new potential airway irritation marker, palate lung nasal epithelial clone protein, in human nasal lavage fluid with two-dimensional electrophoresis and matrix-assisted laser desorption/ionization-time of flight Electrophoresis 22, 1795). SPLUNC1 is expressed in both submucosal glands, the superficial epithelia and in neutrophils and in theory, is present in the correct regions of the lung to be a volume sensing molecule since it can be secreted onto the mucosal surface of the superficial epithelial where ENaC is expressed (Bartlett et al., J. Leukoc. Biol. 83:1201 (2008); Bingle et al., J. Pathol. 205:491 (2005)).
The present invention addresses previous shortcomings in the art by disclosing the regulation of sodium channels by PLUNC proteins and the manipulation of this pathway to regulate sodium absorption and fluid volume and treat disorders responsive to modulating sodium absorption.